diagnostic: silence a Jenkins doctor warning.

Allow our Jenkins CI testing for Homebrew/brew to live outside of
`/usr/local` without complaint (as it's not using any binary packages
anyway).

Library/Homebrew/diagnostic.rb

@@ -407,6 +407,12 @@ module Homebrew
       def check_homebrew_prefix
         return if HOMEBREW_PREFIX.to_s == "/usr/local"
 
+        # Allow our Jenkins CI tests to live outside of /usr/local.
+        if ENV["JENKINS_HOME"] &&
+           ENV["GIT_URL"].to_s.start_with?("https://github.com/Homebrew/brew")
+          return
+        end
+
         <<-EOS.undent
           Your Homebrew's prefix is not /usr/local.
           You can install Homebrew anywhere you want but some bottles (binary packages)

Library/Homebrew/test/diagnostic_test.rb

@@ -84,6 +84,7 @@ class DiagnosticChecksTest < Homebrew::TestCase
   end
 
   def test_check_homebrew_prefix
+    ENV.delete("JENKINS_HOME")
     # the integration tests are run in a special prefix
     assert_match "Your Homebrew's prefix is not /usr/local.",
       @checks.check_homebrew_prefix